✎ Cosmological Constant Problem [The Physics Travel Guide]

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====== Cosmological Constant ======

<blockquote>
From a post-natural perspective, it seems that the problem of the cosmological constant can no longer be ignored. By making a field expansion of the SM effective potential, it appears that the problem with the cosmological constant and the Higgs mass have a common origin. The only difference is that the vacuum energy, unlike the Higgs mass, becomes an observable only through its coupling to gravity. But it seems likely that progress during the post-naturalness era will only come by addressing the two problems simultaneously.
The cosmological constant corresponds to a scale of about 2×10−3 eV and affects physics in the deep IR, at astronomical and cosmological distances. Modifications of gravity at large distances are attempts to tackle the problem from an IR perspective [37]. However, the conceptual problem with the cosmological constant comes from quantum effects in the deep UV. Attempts to tackle the UV problem with the traditional methods used for the Higgs seem hopeless because the corresponding UV cutoff is way lower than any energy scale entering those theories.
This confusion among scales is at the basis of the problem. It is a big source of confusion because the systematic approach of effective field theories has taught us how to separate energy scales in successive shells and make sense of the theory at each shell separately. The cosmological constant seems to resist this approach. Naturalness is an offspring of effective field theory and so it is not surprising that the difficulty we are encountering with the effective theory description leads to a problem with naturalness.
A theory that manifests an active interplay between the IR and the UV would not be simply describable by an effective field theory, as it would violate its inner logic. It is not impossible that quantum gravity will exhibit some kind of IR/UV interplay. An indication could come from the classical behaviour of gravity. Consider the head-on collision of two particles at ever increasing energies. Once you pass the threshold for forming a black hole, the more energy you feed in the system the larger the Schwarzschild radius becomes. In other words, higher energy collisions are less sensitive to short distances, in contrast with our effective-theory intuition for a separation between IR and UV.
A radical conclusion that could be derived from these considerations is that we are facing the end of validity of field theory and the solution of the cosmological constant lies beyond our familiar theories. The new framework should incorporate an interplay of physical effects occurring at all scales. Although it is difficult to tell how such a framework would look, one can easily expect that the cosmological constant problem will play a key role in the post-naturalness era.

<cite>https://arxiv.org/pdf/1710.07663.pdf</cite>

</blockquote>

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Explanations in this section should contain no formulas, but instead colloquial things like you would hear them during a coffee break or at a cocktail party.
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In this section things should be explained by analogy and with pictures and, if necessary, some formulas.
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The motto in this section is: //the higher the level of abstraction, the better//.
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